Lubricating oil composition for continuously variable transmission

ABSTRACT

A lubricating oil composition for a continuously variable transmission including: (A) a lubricant base oil; (B) a borate ester compound in an amount of 25 to 500 mass ppm in terms of boron on the basis of the total mass of the composition; (C) phosphoric acid in an amount of 100 to 750 mass ppm in terms of phosphorus on the basis of the total mass of the composition; (D) a poly(meth)acrylate having a weight average molecular weight of no more than 100,000, wherein the lubricating oil composition has a kinematic viscosity at 40° C. of no more than 25 mm 2 /s.

FIELD

The present invention relates to lubricating oil compositions for continuously variable transmissions, and more specifically, relates to a lubricating oil composition preferable for metal belt type continuously variable transmissions for automobiles.

BACKGROUND

Recent years, it has been demanded that various machines such as automobiles, construction machinery, and agricultural machinery achieve energy efficiency, that is, low fuel consumption, and that devices such as engines and transmissions contribute to energy efficiency.

One means for transmissions to achieve energy efficiency is making lubricating oil less viscous. It is considered that less viscous lubricating oil reduces fluid resistance and drag torque which are caused by viscosity resistance of lubricating oil, and improves power transmission efficiency, and thus makes it possible to improve fuel efficiency.

Another means for transmissions to achieve energy efficiency is making transmissions smaller and lighter. Smaller and lighter transmissions offer improvement of fuel consumption of vehicles on which transmissions are mounted. Particularly, higher friction coefficients between metals allow continuously variable transmissions (for example, metal belt type continuously variable transmissions) to be downsized. Thus, it is desirable that lubricating oil used in continuously variable transmissions keep high friction coefficients between metals.

CITATION LIST Patent Literature

Patent Literature 1: JP 2014-196396 A

Patent Literature 2: JP 4663843 B

SUMMARY Technical Problem

Less viscous lubricating oil leads to reduced oil film thickness on a lubricated surface. Reduced oil film thickness in a mixed lubrication regime is considered to be advantageous in view of increasing friction coefficients between metals. However, reduced oil film thickness on a lubricated surface tends to lead to worsened anti-seizure and anti-wear performance, and a shortened fatigue life.

An object of the present invention is to provide a lubricating oil composition for a continuously variable transmission which achieves improved fuel efficiency and friction coefficients between metals while the composition satisfies anti-seizure performance, anti-wear performance, and a fatigue life which are demanded of continuously variable transmission oil.

Solution to Problem

One embodiment of the present invention is a lubricating oil composition for a continuously variable transmission comprising: (A) a lubricant base oil; (B) a borate ester compound in an amount of 25 to 500 mass ppm in terms of boron (as boron atom content) on the basis of the total mass of the composition; (C) phosphoric acid in an amount of 100 to 750 mass ppm in terms of phosphorus (as phosphorus atom content) on the basis of the total mass of the composition; (D) a poly(meth)acrylate having a weight average molecular weight of no more than 100,000, wherein the lubricating oil composition has a kinematic viscosity at 40° C. of no more than 25 mm²/s.

In this specification, “(meth)acrylate” means “acrylate and/or methacrylate”.

Preferably, the lubricating oil composition for a continuously variable transmission further comprises: (E) a thiadiazole compound in an amount of 180 to 900 mass ppm in terms of sulfur (as sulfur atom content) on the basis of the total mass of the composition, wherein a ratio (B+P)/S of a sum of a boron content B (unit: mass ppm) in the composition derived from the component (B) and a phosphorus content P (unit: mass ppm) in the composition to a sulfur content S (unit: mass ppm) in the composition derived from the component (E) is 1 to 3.

Preferably, in the lubricating oil composition for a continuously variable transmission, the (A) lubricant base oil comprises: (A1) a base oil having a kinematic viscosity at 100° C. of no more than 2.8 mm²/s, a viscosity index of no less than 110, and % C_(P) of no less than 90, in an amount of 30 to 100 mass % on the basis of the total mass of the lubricant base oil, the (A) lubricant base oil optionally comprises: (A2) an API group III base oil or group IV base oil or mixture thereof, having a kinematic viscosity at 100° C. of 3 to 10 mm²/s, in an amount of no more than 70 mass % on the basis of the total mass of the lubricant base oil; and a base oil other than the base oils (A1) and (A2), in an amount of no more than 4 mass % on the basis of the total mass of the lubricant base oil, wherein the (A) lubricant base oil has a kinematic viscosity at 100° C. of no more than 3.4 mm²/s.

Preferably, in the lubricating oil composition for a continuously variable transmission, the component (B) is at least one borate ester compound represented by the following general formula (1):

wherein in the formula (1), R¹ is a hydrocarbyl group having a carbon number of 1 to 30; and R² and R³ are each independently a hydrogen atom or a hydrocarbyl group having a carbon number of 1 to 30.

Advantageous Effects of Invention

The present invention can provide a lubricating oil composition for a continuously variable transmission which achieves improved fuel efficiency and friction coefficients between metals while the composition satisfies anti-seizure performance, anti-wear performance, and a fatigue life which are demanded of continuously variable transmission oil.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention will be described hereinafter. Expression “A to B” concerning numeral values A and B means “no less than A and no more than B” unless otherwise specified. In such expression, if a unit is added only to the numeral value B, the same unit is applied to the numeral value A as well. A word “or” means a logical sum unless otherwise specified.

<(A) Lubricant Base Oil>

A base oil consisting of at least one selected from mineral base oils and synthetic base oils can be used as a lubricant base oil in the lubricating oil composition for a continuously variable transmission of the present invention (hereinafter may be referred to as “continuously variable transmission oil” or “lubricating oil composition”) without any limitation.

Specific examples of mineral base oils include paraffinic or naphthenic mineral base oils obtained through at least one of refining processes such as solvent deasphalting, solvent extraction, hydrocracking, hydroisomerizing, solvent dewaxing, catalytic dewaxing, and hydrorefining on lubricant oil fractions obtained by vacuum distillation of atmospheric residue obtained by atmospheric distillation of crude oil, wax isomerized mineral oils, and base oils produced by a process including isomerizing GTL WAX (gas to liquid wax).

Hydrocracked mineral base oils, and/or wax isomerized isoparaffinic base oils that are obtained by isomerizing raw material containing 50 mass % or more of petroleum wax or GTL wax (for example, Fischer-Tropsch synthetic oil) can be preferably used as mineral base oils.

Examples of synthetic base oils include poly-α-olefins (such as ethylene-propylene copolymer, polybutene, 1-octene oligomer, and 1-decene oligomer) or hydrogenated products thereof; monoesters (such as butyl stearate, and octyl laurate); diesters (such as ditridecyl glutarate, bis(2-ethylhexyl) azipate, diisodecyl azipate, ditridecyl azipate, and bis(2-ethylhexyl) sebacate); polyesters (such as trimellitate esters); polyol esters (such as trimethylolpropane caprylate, trimethylolpropane pelargonate, pentaerythritol 2-ethylhexanoate, and pentaerythritol pelargonate); aromatic synthetic oils (such as alkylbenzene, alkylnaphthalene, and aromatic esters); and mixtures thereof.

Preferably, % C_(P) of a mineral base oil is no less than 70, and more preferably no less than 80; and usually no more than 99, and preferably no more than 95. A mineral base oil having % C_(P) of this lower limit or above makes it possible to improve viscosity-temperature characteristics, thermal and oxidation stability, and friction properties. A mineral base oil having % C_(P) of this upper limit or below makes it possible to improve solubility of additives.

Preferably, % C_(A) of a mineral base oil is no more than 2, more preferably no more than 1, further preferably no more than 0.8, and especially preferably no more than 0.5. A mineral base oil having % C_(A) of this upper limit or below makes it possible to improve viscosity-temperature characteristics, thermal and oxidation stability, and fuel efficiency.

Preferably, % C_(N) of a mineral base oil is no more than 30, and more preferably no more than 25; and preferably no less than 1, and more preferably no less than 4. A mineral base oil having % C_(N) of this upper limit or below makes it possible to improve viscosity-temperature characteristics, thermal and oxidation stability, and friction properties. A mineral base oil having % C_(N) of this lower limit or above makes it possible to improve solubility of additives.

In this specification, % C_(P), % C_(N) and % C_(A) mean percentage of the paraffin carbon number to all the carbon atoms, percentage of the naphthene carbon number to all the carbon atoms, and percentage of the aromatic carbon number to all the carbon atoms, which are obtained by the method conforming to ASTM D 3238-85 (n-d-M ring analysis), respectively. That is, the above described preferred ranges of % C_(P), % C_(N), and % C_(A) are based on values obtained according to the above method. For example, the value of % C_(N) obtained according to the above method can indicate more than 0 even if a mineral base oil does not contain any naphthenes.

The kinematic viscosity of the lubricant base oil at 100° C. is preferably no more than 6.0 mm²/s, more preferably no more than 4.5 mm²/s, and especially preferably no more than 3.4 mm²/s; and preferably no less than 2.0 mm²/s, more preferably no less than 2.5 mm²/s, and especially preferably no less than 2.6 mm²/s. The base oil having kinematic viscosity of this upper limit or below at 100° C. leads to good low-temperature viscosity properties of the lubricating oil composition, and makes it possible to improve fuel efficiency. The base oil having kinematic viscosity of this lower limit or above at 100° C. leads to enough oil film formation at a lubricating point, which makes it possible to improve lubricity. In this specification, “kinematic viscosity at 100° C.” means kinematic viscosity at 100° C., which is specified by ASTM D-445.

The kinematic viscosity of the lubricant base oil at 40° C. is preferably no more than 40 mm²/s, more preferably no more than 30 mm²/s, further preferably no more than 25 mm²/s, and especially preferably no more than 20 mm²/s; and preferably no less than 8.0 mm²/s, more preferably no less than 8.5 mm²/s, and especially preferably no less than 9.0 mm²/s. The lubricant base oil having kinematic viscosity of this upper limit or below at 40° C. leads to good low-temperature viscosity properties of the lubricating oil composition, and makes it possible to improve fuel efficiency. The base oil having kinematic viscosity of this lower limit or above at 40° C. leads to enough oil film formation at a lubricating point, which makes it possible to improve lubricity. In this specification, “kinematic viscosity at 40° C.” means kinematic viscosity at 40° C., which is specified by ASTM D-445.

The viscosity index of the lubricant base oil is preferably no less than 100, more preferably no less than 110, and further preferably no less than 115.

The base oil having viscosity index of this lower limit or above makes it possible to improve viscosity-temperature characteristics, thermal and oxidation stability, and even anti-wear properties. The viscosity index in this specification means a viscosity index measured conforming to JIS K 2283-1993.

The pour point of the lubricant base oil is preferably no more than −10° C., more preferably no more than −12.5° C., further preferably no more than −15° C., especially preferably no more than −17.5° C., and most preferably no more than −20.0° C. The pour point beyond this upper limit tends to lead to deteriorated low-temperature fluidity of whole of the lubricating oil composition. The pour point in this specification means a pour point measured conforming to JIS K 2269-1987.

The sulfur content in the lubricant base oil is, in view of oxidation stability, preferably no more than 1.5 mass %, and more preferably no more than 1.0 mass %.

The following base oil (hereinafter may be referred to as “lubricant base oil according to this embodiment”) is preferably used as the lubricant base oil in the lubricant oil composition of the present invention: the base oil comprising (A1) a base oil having a kinematic viscosity at 100° C. of no more than 2.8 mm²/s, a viscosity index of no less than 110, and % C_(P) of no less than 90, in an amount of 30 to 100 mass % on the basis of the total mass of the lubricant base oil; and optionally comprising (A2) an API group III base oil or group IV base oil or mixture thereof, having a kinematic viscosity at 100° C. of 3 to 10 mm²/s, in an amount of no more than 70 mass % on the basis of the total mass of the lubricant base oil, and a base oil other than the base oils (A1) and (A2), in an amount of no more than 4 mass % on the basis of the total mass of the lubricant base oil; wherein the base oil has a kinematic viscosity at 100° C. of no more than 3.4 mm²/s. Using the lubricant base oil according to this embodiment as the lubricant base oil makes it possible to reduce oil film thickness in a transition regime (mixed lubrication regime) between a hydrodynamic lubrication regime and a boundary lubrication regime, to improve friction coefficients between metals.

The kinematic viscosity of the base oil (A1) at 100° C. is no more than 2.8 mm²/s, and preferably no more than 2.7 mm²/s; and preferably no less than 1.5 mm²/s, and more preferably no less than 2.0 mm²/s. The base oil (A1) having kinematic viscosity of this upper limit or below at 100° C. makes it possible to improve friction coefficients between metals. The base oil (A1) having kinematic viscosity of this lower limit or above at 100° C. leads to enough oil film formation at lubricating points, which makes it possible to improve lubricity.

The kinematic viscosity of the base oil (A1) at 40° C. is preferably no more than 15 mm²/s, and more preferably no more than 10 mm²/s; and preferably no less than 2.0 mm²/s, and more preferably no less than 5.0 mm²/s. The base oil (A1) having kinematic viscosity of this upper limit or below at 40° C. makes it easy to improve friction coefficients between metals. The base oil (A1) having kinematic viscosity of this lower limit or above at 40° C. leads to enough oil film formation at a lubricating point, which makes it possible to improve lubricity.

The viscosity index of the base oil (A1) is no less than 110. The base oil (A1) having viscosity index of 110 or more makes it easy to improve friction coefficients between metals. The upper limit thereof is not restricted, normally no more than 150, and preferably no more than 135.

The base oil (A1) has % C_(P) of no less than 90. The base oil (A1) having % C_(P) of 90 or more leads to good thermal and oxidation stability. The upper limit thereof is not restricted, normally no more than 99, and in view of solubility of additives, preferably no more than 95.

Preferably, the base oil (A1) has % C_(A) of no more than 1, more preferably no more than 0.8, and especially preferably no more than 0.5. The base oil may have % C_(A) of 0. The base oil (A1) having % C_(A) of this upper limit or below makes it possible to improve viscosity-temperature characteristics, thermal and oxidation stability, and fuel efficiency.

A mineral base oil having the above described properties can be used as the base oil (A1) without any limitation.

The kinematic viscosity of the base oil (A2) at 100° C. is 3 to 10 mm²/s, preferably no more than 8.0 mm²/s, more preferably no more than 6.0 mm²/s, and further preferably no more than 4.5 mm²/s. The base oil (A2) having kinematic viscosity of this upper limit or below at 100° C. makes it possible to improve friction coefficients between metals. The base oil (A2) having kinematic viscosity of this lower limit or above at 100° C. leads to enough oil film formation at a lubricating point, which makes it possible to improve lubricity.

The base oil (A2) is a group III base oil or group IV base oil in API classification, or mixture thereof, and preferably a group III base oil. A group III base oil is a mineral base oil having the sulfur content of 0.03 mass % or less, the saturated content of 90 mass % or more, and a viscosity index of 120 or more. Group IV base oils are poly-α-olefins.

The content of the base oil (A1) in the lubricant base oil according to this embodiment is no less than 30 mass %, is preferably no less than 35 mass %, and may be 100 mass %, on the basis of the total mass of the lubricant base oil. In this specification, “the content of the base oil (A1) is 100 mass % on the basis of the total mass of the lubricant base oil” means that the lubricant base oil consists of the base oil (A1).

The content of the base oil (A2) in the lubricant base oil according to this embodiment is no more than 70 mass %, is preferably no more than 65 mass %, and may be 0 mass %, on the basis of the total mass of the lubricant base oil. In this specification, “the content of the base oil (A2) is 0 mass %” means that the lubricant base oil does not contain the base oil (A2).

The content of the base oil other than the base oils (A1) and (A2) in the lubricant base oil according to this embodiment is no more than 4 mass %, is preferably no more than 1 mass %, and may be 0 mass %, on the basis of the total mass of the lubricant base oil. In this specification, “the content of the base oil other than the base oils (A1) and (A2) is 0 mass %” means that the lubricant base oil consists of the base oil (A1) and (optionally) the base oil (A2).

<(B) Borate Ester Compound>

The lubricating oil composition of the present invention contains a borate ester compound (hereinafter may be referred to as “component (B)”).

At least one borate ester compound represented by the following general formula (1) can be used as the component (B).

wherein R¹ is a hydrocarbyl group having a carbon number of 1 to 30; and R² and R³ are each independently a hydrogen atom or a hydrocarbyl group having a carbon number of 1 to 30.

Examples of the hydrocarbyl group include an alkyl group (that may have a ring structure), an alkenyl group (that have a double bond at any position, and may have a ring structure), an aryl group, an alkylaryl group, an alkenylaryl group, an arylalkyl group, and an arylalkenyl group.

Examples of the alkyl group include various linear and branched alkyl groups. Examples of the alkyl group having a ring structure include an alkylcycloalkyl group and a cycloalkylalkyl group. Examples of the cycloalkyl group include cycloalkyl groups having carbon number of 5 to 7 such as cyclopentyl group, cyclohexyl group, and cycloheptyl group. A cycloalkyl ring may be substituted in any position.

Examples of the alkenyl group include various linear and branched alkenyl groups. Examples of the alkenyl group having a ring structure include an alkylcycloalkenyl group, an alkenylcycloalkyl group, a cycloalkenylalkyl group, and a cycloalkenylalkenyl group. The cycloalkyl group is the same as the above. Examples of the cycloalkenyl group include cycloalkenyl groups having carbon number of 5 to 7 such as cyclopentenyl group, cyclohexenyl group, and cycloheptenyl group. A cycloalkenyl ring and a cycloalkyl ring may be substituted in any position.

Examples of the aryl group include phenyl group and naphthyl group. An aryl group may have a hydrocarbyl substituent. In the above described alkylaryl group, alkenylaryl group, arylalkyl group, and arylalkenyl group, an aryl group may be substituted in any position.

The hydrocarbyl group having a carbon number of 1 to 30 in the above general formula (1) is preferably an alkyl or alkenyl group, and more preferably an alkyl group. The above described carbon number is preferably no less than 3, and more preferably no less than 5; and preferably no more than 24, and more preferably no more than 12. The hydrocarbyl group having a carbon number of this lower limit or above makes it possible to improve solubility, and friction coefficients between metals. The hydrocarbyl group having carbon number beyond this upper limit tends to lead to decreased friction coefficients between metals.

In the above general formula (1), preferably at least one of R² and R³ is a hydrogen atom, and more preferably both R² and R³ are hydrogen atoms. Using the component (B) of such an embodiment makes it possible to strengthen a lubricating film in a boundary lubrication regime, and to improve anti-seizure performance and anti-wear performance.

One preferred example of the component (B) is a borate ester compound represented by above general formula (1) wherein R¹ is an alkyl or alkenyl group having a carbon number of 3 to 12, and R² and R³ are hydrogen atoms.

The content of the component (B) in the lubricating oil composition is 25 to 500 mass ppm, preferably no more than 400 mass ppm, and more preferably no more than 300 mass ppm, in terms of boron (as boron atom content) on the basis of the total mass of the composition. The content of the component (B) of this lower limit or above makes it possible to strengthen a lubricating film in a boundary lubrication regime, and to improve anti-seizure performance and anti-wear performance. The content of the component (B) of this upper limit or below makes it possible to strengthen a lubricating film in a boundary lubrication regime, to improve anti-seizure performance, and to lengthen a fatigue life.

<(C) Phosphoric Acid>

The lubricant oil composition of the present invention contains phosphoric acid (hereinafter may be referred to as “component (C)”).

At least one phosphoric acid selected from orthophosphoric acid, pyrophosphoric acid, condensed phosphoric acids, and metaphosphoric acid can be used as the component (C). Orthophosphoric aicd and/or metaphosphoric acid is/are preferable, and orthophosphoric acid is especially preferable as the component (C).

The content of the component (C) in the lubricating oil composition is 100 to 750 mass ppm, preferably no less than 120 mass ppm, and especially preferably no less than 140 mass ppm, in terms of phosphorus (as phosphorus atom content) on the basis of the total mass of the composition. The content of the component (C) of this lower limit or above makes it possible to strengthen a lubricating film in a boundary lubrication regime, and to improve anti-seizure performance and anti-wear performance. The content of the component (C) of this upper limit or below makes it possible to strengthen a lubricating film in a boundary lubrication regime, to improve anti-seizure performance, and to lengthen a fatigue life.

<(D) Poly(meth)acrylate>

The lubricating oil composition of the present invention contains a poly(meth)acrylate having a weight average molecular weight of no more than 100,000 (hereinafter may be referred to as “component (D)”). In this specification, “(meth)acrylate” means “acrylate and/or methacrylate”.

The component (D) may be either a dispersant or non-dispersant poly(meth)acrylate. The weight average molecular weight of the component (D) is no more than 100,000, preferably no more than 80,000, and more preferably no more than 60,000; and preferably no less than 10,000, and more preferably no less than 15,000. The component (D) having a weight average molecular weight of this upper limit or below makes it possible to improve shear stability of the lubricating oil composition. The component (D) having a weight average molecular weight of this lower limit or above makes it easy to improve a viscosity index of the lubricating oil composition.

The component (D) functions as a viscosity index improver. The content of the component (D) in the lubricating oil composition can be a content such that the kinematic viscosity of the lubricating oil composition at 40° C. becomes within the range described later. Specific content of the component (D) varies according to the weight average molecular weight of the component (D). For example, the content may be 5 to 20 mass % on the basis of the total mass (100 mass %) of the lubricating oil composition.

<(E) Thiadiazole Compound>

The lubricating oil composition of the present invention preferably contains at least one thiadiazole compound (hereinafter may be referred to as “component (E)”).

Examples of the component (E) include a 1,3,4-thiadiazole compound represented by the following general formula (2), a 1,2,4-thiadiazole compound represented by the following general formula (3), and a 1,4,5-thiadiazole compound represented by the following general formula (4).

wherein in the general formulae (2) to (4), R⁴ and R⁵ may be either the same or different, and each independently represent hydrogen or a hydrocarbyl group having a carbon number of 1 to 20; and a and b may be either the same or different, and each independently represent an integer of 0 to 8.

A thiadiazole compound represented by any of the above general formulae (2) to (4) and having a hydrocarbyldithio group can be especially preferably used among the above thiadiazole compounds.

The content of the component (E) in the lubricating oil composition is 180 to 900 mass ppm, and preferably 300 to 900 mass ppm, in terms of sulfur (as sulfur atom content) on the basis of the total mass of the composition. The content of the component (E) of this lower limit or above makes it easy to improve anti-seizure performance, anti-wear performance, and friction coefficients between metals, and to lengthen a fatigue life. The content of the component (E) of this upper limit or below makes it easy to improve anti-seizure performance, and to lengthen a fatigue life.

When the lubricating oil composition contains the component (E), the ratio (B+P)/S of the sum of the boron content B (unit: mass ppm) in the composition derived from the component (B) and the total phosphorus content P (unit: mass ppm) in the composition, to the sulfur content S (unit: mass ppm) in the composition derived from the component (E) is preferably 1 to 3, more preferably no less than 1.2, and further preferably no less than 1.4; and more preferably no more than 2.7, further preferably no more than 2.4, further more preferably no more than 2.2, especially preferably no more than 2.0, and most preferably no more than 1.8. The ratio (B+P)/S of this lower limit or above makes it easy to improve anti-seizure performance, and to lengthen a fatigue life. The ratio (B+P)/S of this upper limit or below makes it easy to improve anti-seizure performance, anti-wear performance, and friction coefficients between metals, and to lengthen a fatigue life.

<(F) Metallic Detergent>

In one preferred embodiment, the lubricant oil composition may further contain a metallic detergent (hereinafter may be referred to as “component (F)”).

A known metallic detergent such as a sulfonate detergent, a phenate detergent and a salicylate detergent can be used as the component (F), and (a) sulfonate and/or salicylate detergent(s) can be preferably used.

Preferred examples of a sulfonate detergent include alkaline earth metal salts of alkyl aromatic sulfonic acids obtained by sulfonation of alkylaromatics, and basic or overbased salts thereof. The weight-average molecular weight of the alkylaromatics is preferably 400 to 1500, and more preferably 700 to 1300.

Examples of alkaline earth metals include magnesium, barium, and calcium, and magnesium and calcium are preferable. Examples of alkyl aromatic sulfonic acids include what is called petroleum sulfonic acids and synthetic sulfonic acids. Examples of petroleum sulfonic acids here include sulfonated products of alkylaromatics of lubricant oil fractions derived from mineral oils, and what is called mahogany acid, which is a side product of white oils. Examples of synthetic sulfonic acids include sulfonated products of alkylbenzene having a linear or branched alkyl group, obtained by recovering side products in a manufacturing plant of alkylbenzene, which is raw material of detergents, or by alkylating benzene with a polyolefin. Other examples of synthetic sulfonic acids include sulfonated products of alkylnaphthalenes such as dinonylnaphthalene. Sulfonating agents used when sulfonating these alkylaromatics are not limited. For example, a fuming sulfuric acid or a sulfuric anhydride can be used as a sulfonating agent.

Preferred examples of a phenate detergent include overbased salts of alkaline earth metal salts of compounds having the structure represented by the following general formula (5).

In the formula (5), R⁶ is a C₆-C₂₁ linear or branched, saturated or unsaturated alkyl or alkenyl group; c is a polymerization degree, representing an integer of 1 to 10; A is a sulfide (—S—) group or methylene (—CH₂—) group; and d is an integer of 1 to 3. R⁶ may be combination of at least two different groups.

The carbon number of R⁶ in the formula (5) is preferably 9 to 18, and more preferably 9 to 15. If the carbon number of R⁶ is less than 6, the solubility in the base oil might be poor. On the other hand, if the carbon number of R⁶ is beyond 21, it is difficult to be produced and thermal stability might be poor.

The polymerization degree c in the formula (5) is preferably 1 to 3. The polymerization degree c within this range makes it possible to improve thermal stability.

Preferred examples of a salicylate detergent include alkaline earth metal salicylates, and basic or overbased salts thereof. Preferred examples of alkaline earth metal salicylates include compounds represented by the following general formula (6).

In the formula (6), R⁷ each independently represents a C₁₄-C₃₀ alkyl or alkenyl group. “e” is 1 or 2, and preferably 1. When e=2, R⁷ may be combination of different groups. M is an alkaline earth metal, and preferably calcium or magnesium.

A method for producing an alkaline earth metal salicylate is not restricted, and a known method for producing monoalkylsalicylates can be used. For example, an alkaline earth metal salicylate can be obtained by: making a metal base such as an oxide and hydroxide of an alkaline earth metal react with monoalkylsalicylic acid obtained by alkylating a phenol as starting material with an olefin, and then carboxylating the resultant with carbonic acid gas or the like, monoalkylsalicylic acid obtained by alkylating a salicylic acid as starting material with an equivalent of the olefin, or the like; once converting the above monoalkylsalicylic acid or the like to an alkali metal salt such as a sodium salt and potassium salt, and then performing transmetallation with an alkaline earth metal salt; or the like.

The component (F) may be overbased. A method for obtaining overbased calcium sulfonate, phenate or sulfonate is not limited. For example, an alkaline earth metal sulfonate, phenate, or salicylate is made to react with a base such as calcium hydroxide and magnesium hydroxide in the presence of carbonic acid gas.

When the lubricant oil composition contains the component (F), the content is preferably 100 to 1200 mass ppm, more preferably no less than 200 mass ppm, and more preferably no more than 1000 mass ppm, in terms of metal (as metal element content) on the basis of the total mass of the composition. The content of the component (F) within this range makes it possible to further improve friction properties of wet clutches.

<(G) Ashless Dispersant>

In one preferred embodiment, the lubricating oil composition may further contain an ashless dispersant (hereinafter may be referred to as “component (G)”).

For example, at least one compound selected from the following (G-1) to (G-3) can be used as the component (G):

(G-1) succinimide having at least one alkyl or alkenyl group in its molecule, or a derivative thereof (hereinafter may be referred to as “component (G-1)”);

(G-2) benzylamine having at least one alkyl or alkenyl group in its molecule, or a derivative thereof (hereinafter may be referred to as “component (G-2)”); and

(G-3) polyamine having at least one alkyl or alkenyl group in its molecule, or a derivative thereof (hereinafter may be referred to as “component (G-3)”).

The component (G-1) can be especially preferably used as the component (G).

Examples of succinimide having at least one alkyl or alkenyl group in its molecule among the component (G-1) include compounds represented by the following formula (7) or (8).

In the formula (7), R⁸ is a C₄₀-C₄₀₀ alkyl or alkenyl group; f represents an integer of 1 to 5, and preferably 2 to 4. The carbon number of R⁸ is preferably no less than 60, and preferably no more than 350.

In the formula (8), R⁹ and R¹⁰ are each independently a C₄₀-C₄₀₀ alkyl or alkenyl group, and may be combination of different groups. R⁹ and R₁₀ are especially preferably polybutenyl groups. In addition, g represents an integer of 0 to 4, and preferably 1 to 3. The carbon numbers of R⁹ and R¹⁰ are preferably no less than 60, and preferably no more than 350.

R⁸ to R¹⁰ in the formulae (7) and (8) having carbon numbers of these lower limits or over make it possible to obtain good solubility in the lubricant base oil. On the other hand, R⁸ to R¹⁰ having carbon numbers of these upper limits or below make it possible to improve low-temperature fluidity of the lubricating oil composition.

The alkyl or alkenyl groups (R⁸ to R¹⁰) in the formulae (7) and (8) may be linear or branched. Preferred examples thereof include branched alkyl groups and branched alkenyl groups derived from oligomers of olefins such as propylene, 1-butene, and isobutene, or from co-oligomers of ethylene and propylene. Among them, a branched alkyl or alkenyl group derived from an oligomer of isobutene that is conventionally referred to as polyisobutylene, or a polybutenyl group is most preferable.

A preferred number average molecular weight of the alkyl or alkenyl group (R⁸ to R¹⁰) in the formulae (7) and (8) is 800 to 3500.

Succinimide having at least one alkyl or alkenyl group in its molecule includes so-called monotype succinimide represented by the formula (7) where succinic anhydride terminates only one end of a polyamine chain, and so-called bistype succinimide represented by the formula (8) where succinic anhydride terminates both ends of a polyamine chain. The lubricating oil composition may include either monotype or bistype succinimide, and may include both of them as a mixture.

A method for producing succinimide having at least one alkyl or alkenyl group in its molecule is not limited. For example, such succinimide can be obtained by: making alkyl succinic acid or alkenyl succinic acid obtained by making a compound having a C₄₀-C₄₀₀ alkyl or alkenyl group react with maleic anhydride at 100 to 200° C., react with a polyamine. Here, examples of a polyamine include diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and pentaethylenehexamine.

Examples of benzylamine having at least one alkyl or alkenyl group in its molecule among the component (G-2) include compounds represented by the following formula (9).

In the formula (9), R¹¹ is a C₄₀-C₄₀₀ alkyl or alkenyl group; and h represents an integer of 1 to 5, and preferably 2 to 4. The carbon number of is preferably no less than 60, and preferably no more than 350.

A method for producing the component (G-2) is not limited. Examples of such a method include: making a polyolefin such as a propylene oligomer, polybutene, and an ethylene-α-olefin copolymer, react with a phenol, to give an alkylphenol; and then making formaldehyde, and a polyamine such as diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and pentaethylenehexamine, react with the alkylphenol by Mannich reaction.

Examples of a polyamine having at least one alkyl or alkenyl group in its molecule among the component (G-3) include compounds represented by the following formula (10).

R¹²—NH—(CH₂CH₂NH)_(i)—H   (10)

In the formula (10), R¹² is a C₄₀-C₄₀₀ alkyl or alkenyl group; and i represents an integer of 1 to 5, and preferably 2 to 4. The carbon number of R¹² is preferably no less than 60, and preferably no more than 350.

A method for producing the component (G-3) is not limited. Examples of such a method include: chlorinating a polyolefin such as a propylene oligomer, polybutene, and an ethylene-α-olefin copolymer; and then making ammonia, or a polyamine such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and pentaethylenehexamine react with the chlorinated polyolefin.

Examples of derivatives among the components (G-1) to (G-3) include (i) oxygen-containing organic compound-modified products where a part or all of the residual amino groups and/or imino groups is/are neutralized or amidated by making a C₁-C₃₀ monocarboxylic acid such as fatty acids, a C₂-C₃₀ polycarboxylic acid (such as ethanedioic acid, phthalic acid, trimellitic acid, and pyromellitic acid), an anhydride or ester thereof, a C₂-C₆ alkylene oxide, or a hydroxy(poly)oxyalkylene carbonate react with the above described succinimide, benzylamine or polyamine having at least one alkyl or alkenyl group in its molecule (hereinafter referred to as “the above described nitrogen-containing compound”); (ii) boron-modified products where a part or all of the residual amino groups and/or imino groups is/are neutralized or amidated by making boric acid react with the above described nitrogen-containing compound; (iii) phosphoric acid-modified products where a part or all of the residual amino groups and/or imino groups is/are neutralized or amidated by making the above described nitrogen-containing compound react with phosphoric acid; (iv) sulfur-modified products obtained by making a sulfur compound react with the above described nitrogen-containing compound; and (v) modified products obtained by at least two modifications selected from the oxygen-containing organic compound modification, boron modification, phosphoric acid modification, and sulfur modification, on the above described nitrogen-containing compound in combination. Among these derivatives (i) to (v), a boron-modified product of the (G-1) compound is preferably used in view of making it possible to further improve thermal stability of the lubricating oil composition.

The molecular weight of the component (G) is not restricted, and the weight-average molecular weight thereof is preferably 1000 to 20000.

When the lubricating oil composition contains the component (G), the content thereof is, in terms of nitrogen on the basis of the total mass of the lubricating oil composition, preferably 100 to 2000 mass ppm, more preferably no less than 500 mass ppm, and more preferably no more than 1000 mass ppm. The content of the component (G) of this lower limit or over makes it possible to improve anti-coking performance (thermal stability) of the lubricating oil composition. The content of the component (G) of this upper limit or below makes it possible to further improve fuel efficiency.

When a boron-modified product is used as the component (G), the boron content derived from the component (G) in the lubricating oil composition is, on the basis of the total mass of the lubricating oil composition, preferably 50 to 500 mass ppm, more preferably no less than 100 mass ppm, and more preferably no more than 300 mass ppm. The boron content derived from the component (G) of this upper limit or below makes it possible to further improve fuel efficiency.

<(H) Phosphorus-based Anti-wear Agent>

In one preferred embodiment, the lubricating oil composition may further contain a phosphorus-based anti-wear agent (hereinafter may be referred to as “component (H)”). As the component (H), one agent may be used alone or at least two agents may be used in combination.

Examples of the component (H) include zinc dialkyldithophosphate, phosphoric acid monoesters, phosphoric acid diesters, phosphoric acid triesters, phosphorous acid monoesters, phosphorous acid diesters, phosphorous acid triesters, salts of phosphoric acid incomplete esters, salts of phosphorous acid incomplete esters, and mixture thereof.

In the above examples, compounds other than phosphorous acid are usually compounds having hydrocarbon groups of C₂ to C_(30,) preferably C₃ to C₂₀. Specific examples of these hydrocarbon groups of C₂ to C₃₀ include an alkyl group, a cycloalkyl group, an alkyl-substituted cycloalkyl group, an alkenyl group, an aryl group, an alkyl-substituted aryl group, and aryl-substituted alkyl group. These alkyl groups may be either linear or branched.

Examples of the above described salts of phosphoric acid incomplete esters and salts of phosphorous acid incomplete esters include salts where a part or all of the residual acidic hydrogen is/are neutralized by making a nitrogen-containing compound such as ammonia and amine compounds containing only a C₁ to C₈ hydrocarbon group or C₁ to C₈ hydroxy group-containing hydrocarbon group in their molecules react with phosphoric acid monoester, phosphoric acid diester, phosphorous acid monoester, or phosphorous acid diester, or mixture thereof.

Phosphorous acid ester or its salt can be preferably used as the component (H). Phosphorous acid incomplete ester or its salt is more preferable, and phosphorous acid incomplete ester or its salt having a hydrocarbon group having 8 or less carbon atoms is especially preferable.

When the lubricating oil composition contains the component (H), the content thereof is preferably 100 to 2000 mass ppm, more preferably no less than 200 mass ppm, and more preferably no more than 1000 mass ppm, in terms of phosphorus on the basis of the total mass of the composition. The content of the component (H) of this lower limit or above makes it possible to further improve anti-wear performance and friction coefficients between metals. The content of the component (H) of this upper limit or below makes it possible to improve oxidation stability and compatibility with sealing.

<(I) Friction Modifier>

In one preferred embodiment, the lubricating oil composition may further contain a friction modifier (hereinafter may be referred to as “component (I)”). As the component (I), one friction modifier may be used alone or at least two friction modifiers may be used in combination.

As the component (I), a compound used as a friction modifier in the field of lubricant oil can be used without particular limitation. Examples of a friction modifier include C₆-C₅₀ compounds containing, in their molecules, at least one heteroatom selected from an oxygen atom, a nitrogen atom and a sulfur atom. More specifically, any ashless friction modifier such as aliphatic amine compounds; aliphatic imide compounds; and fatty acid esters, fatty acid amides, fatty acid hydrazides, fatty acid metal salts, aliphatic alcohols, aliphatic ethers, and aliphatic urea compounds, each having at least one C₆-C₃₀ linear or branched alkyl or alkenyl group in its molecule, can be preferably used.

Examples of aliphatic amine compounds include C₆-C₃₀ linear or branched, preferably linear aliphatic monoamines; C₆-C₃₀ linear or branched, preferably linear aliphatic polyamines; and alkylene oxide adducts of these aliphatic amines.

Examples of aliphatic imide compounds include succinimide having a C₆-C₃₀ linear or branched alkyl or alkenyl group; and modified products thereof by carboxylic acids, boric acid, phosphoric acid, or sulfuric acid.

Examples of fatty acid esters include esters of C₆-C₃₀ linear or branched, preferably linear fatty acids, and aliphatic monoalcohols or aliphatic polyols.

Examples of fatty acid amides include amides of C₆-C₃₀ linear or branched, preferably linear fatty acids, and aliphatic monoamines or aliphatic polyamines, or ammonia.

Examples of fatty acid hydrazides include condensation products of C₆-C₃₀ linear or branched, preferably linear fatty acids, and unsubstituted or substituted aliphatic hydrazines.

Examples of fatty acid metal salts include alkaline earth metal salts (such as a magnesium salt and a calcium salt) and a zinc salt of C₆-C₃₀ linear or branched, preferably linear fatty acids.

When the lubricating oil composition contains the component (I), the content thereof is preferably 0.01 to 2 mass %, more preferably no less than 0.1 mass %, and further preferably no less than 0.3 mass %; and more preferably no more than 1.0 mass %, and further preferably no more than 0.8 mass %, on the basis of the total mass of the lubricating oil composition. The content of the component (I) of this lower limit or over makes it possible to improve shudder prevention performance. The content of the component (I) of this upper limit or below makes it possible to further improve friction coefficients between metals.

<Other Additives>

In one embodiment, the lubricating oil composition may further contain at least one additive selected from anti-wear agents or extreme-pressure agents other than the component (H), antioxidants, pour point depressants other than the component (D), corrosion inhibitors other than the component (E), anti-rust agents, metal deactivators other than the component (E), defoaming agents, demulsifiers, and coloring agents.

Examples of anti-wear agents or extreme-pressure agents other than the component (H) include sulfur-based compounds such as disulfides, sulfurized olefins, and sulfurized oils. When the lubricating oil composition contains an anti-wear agent or extreme-pressure agent other than the component (H), the content thereof is usually 0.01 to 5 mass % on the basis of the total mass of the lubricating oil composition.

Examples of antioxidants include phenolic or amine ashless antioxidants, and copper or molybdenum metallic antioxidants. Specific examples of phenolic ashless antioxidants include 4,4′-methylenebis(2,6-di-tert-butylphenol), and 4,4′-bis(2,6-di-tert-butylphenol); and examples of amine ashless antioxidants include phenyl-α-naphthylamine, alkylphenyl-α-naphthylamine, and dialkyldiphenylamine. When the lubricating oil composition contains an antioxidant, the content thereof is usually 0.01 to 5 mass % on the basis of the total mass of the lubricating oil composition.

For example, a known pour point depressant such as a polymethacrylate polymer can be used as a pour point depressant other than the component (D), according to properties of the lubricant base oil to be used. When the lubricating oil composition contains a pour point depressant, the content thereof is usually 0.05 to 1 mass % on the basis of the total mass of the lubricating oil composition.

A known corrosion inhibitor such as a benzotriazole, tolyltriazole, and imidazole compound can be used as a corrosion inhibitor other than the component (E). When the lubricating oil composition contains a corrosion inhibitor other than the component (E), the content thereof is usually 0.005 to 5 mass % on the basis of the total mass of the lubricating oil composition.

A known anti-rust agent such as petroleum sulfonate, alkylbenzenesulfonate, dinonylnaphthalenesulfonate, alkenylsuccinate esters, and polyol esters can be used as an anti-rust agent. When the lubricating oil composition contains an anti-rust agent, the content thereof is usually 0.005 to 5 mass % on the basis of the total mass of the lubricating oil composition.

A known metal deactivator such as imidazoline, pyrimidine derivatives, mercaptobenzothiazole, benzotriazole and their derivatives, 2-(alkyldithio)benzimidazole, and β-(o-carboxybenzylthio)propionitrile can be used as a metal deactivator other than the component (E). When the lubricating oil composition contains a metal deactivator other than the component (E), the content thereof is usually 0.005 to 5 mass % on the basis of the total mass of the lubricating oil composition.

A known anti-foaming agent such as silicones, fluorosilicones, and fluoroalkyl ethers can be used as an anti-foaming agent. When the lubricating oil composition contains an anti-foaming agent, the content thereof is usually 0.0005 to 0.01 mass % on the basis of the total mass of the lubricating oil composition.

A known demulsifier such as polyalkylene glycol-based nonionic surfactants can be used as a demulsifier. When the lubricating oil composition contains a demulsifier, the content thereof is usually 0.005 to 5 mass % on the basis of the total mass of the lubricating oil composition.

As a coloring agent, for example, a known coloring agent such as azo compounds can be used.

<Lubricating Oil Composition>

The kinematic viscosity of the lubricating oil composition at 40° C. is no more than 25 mm²/s, and preferably no less than 10 mm²/s, more preferably no less than 12 mm²/s, and further preferably no less than 15 mm²/s. When the kinematic viscosity of the lubricating oil composition at 40° C. is no more than 25 mm²/s, fuel efficiency can be improved. The kinematic viscosity of the lubricating oil composition at 40° C. of this lower limit or above makes it easy to achieve enough oil film formation at a lubricating point, to improve anti-wear performance.

The kinematic viscosity of the lubricating oil composition at 100° C. is preferably no less than 5.0 mm²/s, and preferably no more than 9.0 mm²/s, more preferably no more than 8.0 mm²/s, and further preferably no more than 7.0 mm²/s. The kinematic viscosity of the lubricating oil composition at 100° C. of this lower limit or above makes it easy to achieve enough oil film formation at a lubricating point, to improve anti-wear performance. When the kinematic viscosity thereof is this upper limit or below, fuel efficiency can be easily improved.

The viscosity index of the lubricating oil composition is preferably no less than 170. The upper limit of the viscosity index of the lubricating oil composition is not restricted, and is usually no more than 300. The lubricating oil composition having a viscosity index of 170 or more makes it easy to improve fuel efficiency.

Brookfield viscosity (hereinafter may be referred to as “BF viscosity”) of the lubricating oil composition at −40° C. is preferably no more than 8,000 mPa·s, and more preferably no more than 7,000 mPa·s. The lubricating oil composition having BF viscosity of no more than 8,000 mPa·s at −40° C. makes it possible to improve low-temperature startability.

(Use)

The lubricating oil composition of the present invention can be preferably used as continuously variable transmission oil for automobiles, and especially preferably used for lubrication of metal belt type continuously variable transmissions where torque is transmitted through a metal belt.

EXAMPLES

Hereinafter the present invention will be more specifically described based on the examples and comparative examples. It is noted that the present invention is not limited to these examples.

Examples 1 to 19 and Comparative Examples 1 to 7

The lubricating oil compositions of the present invention (examples 1 to 19) and the lubricating oil compositions for comparison (comparative examples 1 to 7) were prepared as shown in Tables 1 to 3. In Tables, the contents of the base oils are on the basis of the total mass of the base oils, and the content of each additive is on the basis of the total mass of the composition. Details on the components were as follows.

((A) Lubricant Base Oil)

A1-1: wax-isomerized base oil (kinematic viscosity (40° C.): 9.072 mm²/s, kinematic viscosity (100° C.): 2.621 mm²/s, viscosity index: 127, sulfur content: less than 10 mass ppm, % C_(P): 91.8, % C_(N): 8.2, % C_(A): 0)

A1-2: wax-isomerized base oil (kinematic viscosity (40° C.): 9.617 mm²/s, kinematic viscosity (100° C.): 2.653 mm²/s, viscosity index: 112, sulfur content: less than 10 mass ppm, % C_(P): 92.5, % C_(N): 7.3, % C_(A): 0.1)

A2-1: wax-isomerized base oil (kinematic viscosity (40° C.): 15.65 mm²/s, kinematic viscosity (100° C.): 3.883 mm²/s, viscosity index: 142, sulfur content: less than 10 mass ppm, % C_(P): 92.5, % C_(N): 7.5, % C_(A): 0)

A2-2: wax-isomerized base oil (kinematic viscosity (40° C.): 18.24 mm²/s, kinematic viscosity (100° C.): 4.119 mm²/s, viscosity index: 130, sulfur content: less than 10 mass ppm, % C_(P): 89.1, % C_(N): 10.9, % C_(A): 0)

((B) Borate Ester Compound)

B-1: boric acid mono(C₆-C₈ alkyl)ester, B: 2.83 mass %

((C) Phosphoric Acid)

C-1: phosphoric acid, P: 36 mass %

((D) Poly(meth)acrylate)

-   -   D-1: non-dispersant polymethacrylate, weight average molecular         weight: 20,000

D-2: non-dispersant polymethacrylate, weight average molecular weight: 50,000

D-3: non-dispersant polymethacrylate, weight average molecular weight: 40,000

D-4: non-dispersant polymethacrylate, weight average molecular weight: 150,000

((E) Thiadiazole Compound)

E-1: thiadiazole compound represented by any of the general formulae (2) to (4) having a hydrocarbyldithio group, S: 36 mass %

((F) Metallic Detergent)

F-1: Ca sulfonate, Ca: 18.4 mass %

F-2: Ca salicylate, Ca: 8.1 mass %

((G) Ashless Dispersant)

G-1: succinimide, N: 2.1 mass %

G-1: boron-containing succinimide, N: 2.3 mass %, B: 2.0 mass %

((H) Phosphorus-based Anti-wear Agent)

H-1: di(n-butyl) phosphite, P: 15.5 mass %

H-2: diphenyl hydrogen phosphite, P: 13.2 mass %

((I) Friction Modifier)

I-1: oleylamine

I-2: oleylamide

I-3: condensation product of a fatty acid and an aliphatic monoamine, carbon number: 67 to 86

((J) Other Additives)

J-1: rubber swelling agent

J-2: amine antioxidant

J-3: dimethylsilicone anti-foaming agent, kinematic viscosity (25° C.): 60,000 mm²/s

TABLE 1 Examples 1 2 3 4 5 (A) Base oil A1-1 mass % (80) — (83) — (36) A1-2 mass % — (85) — (83) — A2-1 mass % (20) — — (17) (64) A2-2 mass % — (15) (17) — — viscosity characteristics of base oil kinematic viscosity (40° C.) mm²/s 10.0 10.5 10.1 10.4 12.7 kinematic viscosity (100° C.) mm²/s 2.82 2.82 2.81 2.82 3.34 viscosity index 132 115 126 118 140 (B) Borate ester compound B-1 mass % 0.50 0.50 0.50 0.50 0.50 (content “B” in terms of boron) mass ppm 142 142 142 142 142 (C) Phosphoric acid C-1 mass % 0.10 0.10 0.10 0.10 0.10 (in terms of phosphorus) mass ppm 300 300 300 300 300 (D) poly(meth)acrylate D-1 mass % 18.5 18.5 18.5 18.5 14.3 D-2 mass % — — — — — D-3 mass % — — — — — D-4 mass % — — — — — (E) Thiadiazole compound E-1 mass % 0.15 0.15 0.15 0.15 0.15 (content “S” in terms of sulfur) mass ppm 540 540 540 540 540 (F) Metallic detergent F-1 mass % 0.22 0.22 0.22 0.22 0.22 F-2 mass % — — — — — (G) Ashless dispersant G-1 mass % 3.0 3.0 3.0 3.0 3.0 G-2 mass % 0.5 0.5 0.5 0.5 0.5 (H) Phosphorus-based Anti- wear agent H-1 mass % 0.30 0.30 0.30 0.30 0.30 H-2 mass % — — — — — (I) Friction modifier I-1 mass % 0.03 0.03 0.03 0.03 0.03 I-2 mass % 0.05 0.05 0.05 0.05 0.05 I-3 mass % 0.50 0.50 0.50 0.50 0.50 (J) Other additives Rubber swelling agent (J-1) mass % 0.6 0.6 0.6 0.6 0.6 Antioxidant (J-2) mass % 0.5 0.5 0.5 0.5 0.5 Anti-foaming agent (J-3) mass % 0.003 0.003 0.003 0.003 0.003 Phosphorus content “P” in mass ppm 765 765 765 765 765 composition (B + P)/S 1.7 1.7 1.7 1.7 1.7 viscosity characteristics of composition kinematic viscosity (40° C.) mm²/s 21.6 22.4 21.9 22.0 22.3 kinematic viscosity (100° C.) mm²/s 5.35 5.31 5.33 5.26 5.41 viscosity index 199 184 193 185 194 BF viscosity (−40° C.) mPa · s 4200 3800 4000 3850 4850 Shear stability test (reduction % 1.8 1.7 1.7 1.8 1.4 of kinematic viscosity) EHL test (oil film thickness) nm 50.1 50.2 50.1 50.1 53.1 High-speed four-ball test LNSL N 618 618 618 618 618 Wear mark size mm 0.53 0.55 0.52 0.54 0.51 FALEX seizure test (load capacity) N 4359 4226 4404 4359 4493 Unisteel test (fatigue life L50) min 1435 1325 1411 1428 1568 LFW-1 test (friction coefficients 0.098 0.097 0.095 0.097 0.093 between metals) Examples 6 7 8 9 10 (A) Base oil A1-1 (100) (80) (80) (80) (80) A1-2 — — — — — A2-1 — (20) (20) (20) (20) A2-2 — — — — — viscosity characteristics of base oil kinematic viscosity (40° C.) 9.1 10.0 10.0 10.0 10.0 kinematic viscosity (100° C.) 2.62 2.82 2.82 2.82 2.82 viscosity index 125 132 132 132 132 (B) Borate ester compound B-1 0.50 0.10 1.00 0.50 0.50 (content “B” in terms of boron) 142 28 283 142 142 (C) Phosphoric acid C-1 0.10 0.10 0.10 0.05 0.20 (in terms of phosphorus) 300 300 300 150 600 (D) poly(meth)acrylate D-1 19.2 18.5 18.5 18.5 18.5 D-2 — — — — — D-3 — — — — — D-4 — — — — — (E) Thiadiazole compound E-1 0.15 0.15 0.15 0.15 0.15 (content “S” in terms of sulfur) 540 540 540 540 540 (F) Metallic detergent F-1 0.22 0.22 0.22 0.22 0.22 F-2 — — — — — (G) Ashless dispersant G-1 3.0 3.0 3.0 3.0 3.0 G-2 0.5 0.5 0.5 0.5 0.5 (H) Phosphorus-based Anti- wear agent H-1 0.30 0.30 0.30 0.30 0.30 H-2 — — — — — (I) Friction modifier I-1 0.03 0.03 0.03 0.03 0.03 I-2 0.05 0.05 0.05 0.05 0.05 I-3 0.50 0.50 0.50 0.50 0.50 (J) Other additives Rubber swelling agent (J-1) 0.6 0.6 0.6 0.6 0.6 Antioxidant (J-2) 0.5 0.5 0.5 0.5 0.5 Anti-foaming agent (J-3) 0.003 0.003 0.003 0.003 0.003 Phosphorus content “P” in 765 765 765 615 1065 composition (B + P)/S 1.7 1.5 1.9 1.4 2.2 viscosity characteristics of composition kinematic viscosity (40° C.) 21.2 21.6 22.0 21.0 21.1 kinematic viscosity (100° C.) 5.23 5.34 5.42 5.24 5.26 viscosity index 195 198 199 199 199 BF viscosity (−40° C.) 3650 4020 4350 3980 4400 Shear stability test (reduction 1.9 1.7 1.8 1.7 1.8 of kinematic viscosity) EHL test (oil film thickness) 49.2 50.1 50.2 50.2 50.0 High-speed four-ball test LNSL 618 618 618 618 618 Wear mark size 0.48 0.52 0.54 0.62 0.54 FALEX seizure test (load capacity) 4226 4048 4448 4003 4359 Unisteel test (fatigue life L50) 1411 1524 1395 1536 1389 LFW-1 test (friction coefficients 0.101 0.095 0.100 0.094 0.099 between metals)

TABLE 2 Examples 11 12 13 14 15 16 17 18 19 (A) Base oil A1-1 mass % (80) (80) (80) (80) (80) (80) (80) (80) (80) A1-2 mass % — — — — — — — — — A2-1 mass % (20) (20) (20) (20) (20) (20) (20) (20) (20) A2-2 mass % — — — — — — — — — viscosity characteristics of base oil kinematic viscosity (40° C.) mm²/s 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 kinematic viscosity (100° C.) mm²/s 2.82 2.82 2.82 2.82 2.82 2.82 2.82 2.82 2.82 viscosity index 132 132 132 132 132 132 132 132 132 (B) Borate ester compound B-1 mass % 0.50 0.50 0.10 1.00 0.50 0.50 0.50 0.50 0.50 (content “B” in terms of mass ppm 142 142 28 283 142 142 142 142 142 boron) (C) Phosphoric acid C-1 mass % 0.10 0.10 0.05 0.25 0.10 0.10 0.10 0.10 0.10 (in terms of phosphorus) mass ppm 300 300 150 750 300 300 300 300 300 (D) poly(meth)acrylate D-1 mass % 18.5 18.5 18.5 18.5 — — 18.5 18.5 18.5 D-2 mass % — — — — 6.2 — — — — D-3 mass % — — — — — 10.5 — — — D-4 mass % — — — — — — — — — (E) Thiadiazole compound E-1 mass % 0.10 0.25 0.10 0.25 0.15 0.15 0.15 0.15 0.15 (content “S” in terms of mass ppm 360 900 360 900 540 540 540 540 540 sulfur) (F) Metallic detergent F-1 mass % 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.11 F-2 mass % — — — — — — — 0.49 0.25 (G) Ashless dispersant G-1 mass % 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 G-2 mass % 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 (H) Phosphorus-based Anti- wear agent H-1 mass % 0.30 0.30 0.30 0.30 0.30 0.30 — 0.30 0.30 H-2 mass % — — — — — — 0.35 — — (I) Friction modifier I-1 mass % 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 I-2 mass % 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 I-3 mass % 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 (J) Other additives Rubber swelling agent (J-1) mass % 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 Antioxidant (J-2) mass % 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Anti-foaming agent (J-3) mass % 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 Phosphorus content “P” in mass ppm 765 765 615 1215 765 765 762 765 765 composition (B + P)/S 2.5 1.0 1.8 1.7 1.7 1.7 1.7 1.7 1.7 viscosity characteristics of composition kinematic viscosity (40° C.) mm²/s 21.4 21.3 21.0 22.0 24.3 22.8 21.6 21.4 21.6 kinematic viscosity (100° C.) mm²/s 5.31 5.29 5.24 5.43 5.48 5.41 5.35 5.33 5.36 viscosity index 198 198 199 200 173 187 199 200 200 BF viscosity (−40° C.) mPa · s 4130 4300 4100 4540 4250 4100 4200 4180 4210 Shear stability test (reduction % 1.7 1.8 1.7 1.8 2.1 2.0 1.7 1.7 1.7 of kinematic viscosity) EHL test (oil film thickness) nm 50.1 50.1 50.1 50.2 49.8 51.2 50.1 50.2 50.1 High-speed four-ball test LNSL N 618 785 618 618 618 618 618 618 618 Wear mark size mm 0.65 0.52 0.63 0.56 0.61 0.51 0.52 0.59 0.57 FALEX seizure test (load N 3959 4671 4270 4359 4404 4448 4226 4359 4448 capacity) Unisteel test (fatigue life L50) min 1311 1458 1462 1395 1298 1485 1497 1386 1415 LFW-1 test (friction coeffi- 0.094 0.096 0.095 0.096 0.099 0.095 0.096 0.093 0.096 cients between metals)

TABLE 3 Comparative Examples 1 2 3 4 5 6 7 (A) Base oil A1-1 mass % (80) (80) (80) (80) (80) (80) (80) A1-2 mass % — — — — — — — A2-1 mass % (20) (20) (20) (20) (20) (20) (20) A2-2 mass % — — — — — — — viscosity characteristics of base oil kinematic viscosity (40° C.) mm²/s 10.0 10.0 10.0 10.0 10.0 10.0 10.0 kinematic viscosity (100° C.) mm²/s 2.82 2.82 2.82 2.82 2.82 2.82 2.82 viscosity index 132 132 132 132 132 132 132 (B) Borate ester compound B-1 mass % 0.05 2.00 0.50 0.50 0.00 2.00 0.50 (content “B” in terms of mass ppm 14 566 142 142 0 566 142 boron) (C) Phosphoric acid C-1 mass % 0.10 0.10 0.01 0.30 0.00 0.30 0.10 (in terms of phosphorus) mass ppm 300 300 30 900 0 900 300 (D) poly(meth)acrylate D-1 mass % 18.5 18.5 18.5 18.5 18.5 18.5 — D-2 mass % — — — — — — — D-3 mass % — — — — — — — D-4 mass % — — — — — — 4.5 (E) Thiadiazole compound E-1 mass % 0.15 0.15 0.15 0.15 0.01 0.30 0.15 (content “S” in terms of mass ppm 540 540 540 540 36 1080 540 sulfur) (F) Metallic detergent F-1 mass % 0.22 0.22 0.22 0.22 0.22 0.22 0.22 F-2 mass % — — — — — — — (G) Ashless dispersant G-1 mass % 3.0 3.0 3.0 3.0 3.0 3.0 3.0 G-2 mass % 0.5 0.5 0.5 0.5 0.5 0.5 0.5 (H) Phosphorus-based Anti- wear agent H-1 mass % 0.30 0.30 0.30 0.30 0.30 0.30 0.30 H-2 mass % — — — — — — — (I) Friction modifier I-1 mass % 0.03 0.03 0.03 0.03 0.03 0.03 0.03 I-2 mass % 0.05 0.05 0.05 0.05 0.05 0.05 0.05 I-3 mass % 0.50 0.50 0.50 0.50 0.50 0.50 0.50 (J) Other additives Rubber swelling agent (J-1) mass % 0.6 0.6 0.6 0.6 0.6 0.6 0.6 Antioxidant (J-2) mass % 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Anti-foaming agent (J-3) mass % 0.003 0.003 0.003 0.003 0.003 0.003 0.003 Phosphorus content “P” in mass ppm 765 765 495 1365 465 1365 765 composition (B + P)/S 1.4 2.5 1.2 2.8 12.9 1.8 1.7 viscosity characteristics of composition kinematic viscosity (40° C.) mm²/s 21.4 21.6 21.3 21.4 21.1 22.1 25.9 kinematic viscosity (100° C.) mm²/s 5.31 5.34 5.30 5.32 5.26 5.45 5.64 viscosity index 198 198 199 199 199 200 167 BF viscosity (−40° C.) mPa · s 4220 4180 4080 4430 4000 4400 4600 Shear stability test (reduction % 1.8 1.7 1.7 1.8 1.7 1.8 10.3 of kinematic viscosity) EHL test (oil film thickness) nm 50.2 50.2 50.1 50.3 50.1 50.1 50.1 High-speed four-ball test LNSL N 618 785 618 618 489 785 618 Wear mark size mm 0.74 0.44 0.79 0.41 0.83 0.55 0.54 FALEX seizure test (load N 3826 3648 3470 3381 3069 4092 4359 capacity) Unisteel test (fatigue life L50) min 1285 1110 1524 1098 998 1044 1358 LFW-1 test (friction coeffi- 0.093 0.099 0.095 0.097 0.087 0.097 0.095 cients between metals)

(Low-temperature Viscosity Characteristics)

For each lubricating oil composition, viscosity (BF viscosity) at −40° C. in oil temperature was measured using a Brookfield viscometer. The results are shown in Tables 1 to 3. It can be determined that low-temperature viscosity characteristics are good when the BF viscosity at −40° C. is no more than 8,000 mPa·s.

(Shear Stability Test)

For each lubricating oil composition, shear stability of the lubricating oil composition was evaluated by a shear stability test conforming to JPI-5S-29-88. Sample oil was irradiated with ultrasonic waves of 10 kHz in frequency from an oscillator of 28 μm in amplitude for 1 hour, and the reduction (%) of the kinematic viscosity of the sample oil at 100° C. after irradiation of ultrasonic waves, to that before irradiation of ultrasonic waves was calculated. The results are shown in Tables 1 to 3. It can be determined that shear stability is good when the reduction of the kinematic viscosity in this test is no more than 2.5.

(EHL Test)

For each lubricating oil composition, oil film thickness under an elastohydrodynamic lubrication condition was measured by optical interferometry using EHL test instrument (EHD2 ultra thin film measurement system manufactured by PCS Instruments). The measurement conditions were as follows.

steel ball: Standard Ball (material: SUJ-2) manufactured by PCS Instruments, diameter: 19.05 mm

disc: glass disc having a glass substrate, a Cr layer coated on the surface of the glass substrate, and a silica layer coated on the surface of the Cr layer

oil temperature: 80° C.

load: 20 N

average Hertz contact pressure: 0.5 GPa

speed: 1 m/s

slide ratio: 10% The results are shown in Tables 1 to 3. It can be determined that oil film thickness is sufficiently thin when the oil film thickness measured in this test is no more than 55 nm.

(High-speed Four-ball Test)

For each lubricating oil composition, load capacity and anti-wear performance of the lubricating oil composition were evaluated by a high-speed four-ball test conforming to JPI-5S-40-93.

(1) Last non-seizure load (LNSL) was measured at 1800 rpm in rotation speed.

(2) the size of a wear mark after driving at 1200 rpm in rotation speed at 392 N in load and 80° C. in oil temperature for 30 minutes was measured. The results are shown in Tables 1 to 3. It can be determined that load capacity (anti-seizure performance) is good when LNSL is no less than 618 N in this test, and it can be determined that anti-wear performance is good when the size of a wear mark is no more than 0.70 mm in this test.

(FALEX Seizure Test)

For each lubricating oil composition, load capacity was evaluated by a FALEX seizure test conforming to ASTM D3233. Under the condition of oil temperature at 110° C., a steel pin that was sandwiched by two stationary steel V-shaped blocks was rotated at 290 rpm, and the load at which seizure occurred was measured. The results are shown in Tables 1 to 3. It can be determined that load capacity (anti-seizure performance) is good when the load at which seizure occurs is no less than 3900 N in this test.

(Unisteel Test)

For each lubricating oil composition, a rolling fatigue life of a thrust bearing was measured by a Unisteel test (IP305/79, The Institute of Petroleum) using a Unisteel rolling fatigue testing machine (triple-type high-temperature rolling fatigue testing machine (TRF-1000/3-01H) manufactured by Tokyo Koki Testing Machine Co. Ltd.). Time until either a roller or a test piece suffers fatigue damage was measured for a test bearing made by replacing a bearing ring in one side of a thrust needle bearing (FNTA-2542C manufactured by NSK Ltd.) with a flat test piece (material: SUJ2), under conditions of: 7000 N in load; 2 GPa in surface pressure; 1450 rpm in rotation speed; and 120° C. in oil temperature. It was determined that fatigue damage occurred when the vibration acceleration of a testing portion measured by a vibration accelerometer installed in the Unisteel rolling fatigue testing machine reached 1.5 m/s². The test was repeated ten times, and then a fatigue life was calculated as the 50% life (L50: time for the cumulative probability to be 50%) by a Weibull plot based on the time it had taken for fatigue damage to occur in the tests. The results are shown in Tables 1 to 3. It can be determined that a fatigue life is good when the 50% life measured in this test is no less than 1200 minutes.

(LFW-1 Test)

For each lubricating oil composition, friction coefficients between metals (coefficient of dynamic friction) was measured conforming to JASO M358-2005 (Standard Test Method for Metal on Metal Friction Characteristics of Belt CVT Fluids, under high-load condition) with a block-on-ring friction and wear testing machine (LFW-1). The test conditions were as follows; block: H60, ring: S10, load: 1112 N, slide speed: 0.5 m/s, and oil temperature: 90° C. The results are shown in Tables 1 to 3. It can be determined that friction coefficients between metals is sufficiently high when the friction coefficients between metals measured in this test is no less than 0.90.

(Evaluation Results)

The lubricating oil compositions of examples 1 to 19 showed good results in low-temperature viscosity characteristics, shear stability, oil film thickness, load capacity (anti-seizure performance), anti-wear performance, a fatigue life, and a friction coefficient between metals.

The lubricating oil composition of comparative example 1, where the content of the component (B) was too low, was inferior in anti-wear performance and anti-seizure performance.

The lubricating oil composition of comparative example 2, where the content of the component (B) was too high, was inferior in anti-seizure performance and a fatigue life.

The lubricating oil composition of comparative example 3, where the content of the component (C) was too low, was inferior in anti-wear performance and anti-seizure performance.

The lubricating oil composition of comparative example 4, where the content of the component (C) was too high, was inferior in anti-seizure performance and a fatigue life.

The lubricating oil composition of comparative example 5, which did not contain the component (B) or (C), was inferior in anti-seizure performance, anti-wear performance, a fatigue life, and a friction coefficient between metals.

The lubricating oil composition of comparative example 6, where the contents of the components (B) and (C) were too high, was inferior in a fatigue life.

The lubricating oil composition of comparative example 7, where a polymethacrylate out of the range of the component (D) was incorporated instead of the component (D), was inferior in shear stability. 

1. A lubricating oil composition for a continuously variable transmission comprising: (A) a lubricant base oil; (B) a borate ester compound in an amount of 25 to 500 mass ppm in terms of boron on the basis of the total mass of the composition; (C) phosphoric acid in an amount of 100 to 750 mass ppm in terms of phosphorus on the basis of the total mass of the composition; (D) a poly(meth)acrylate having a weight average molecular weight of no more than 100,000, wherein the lubricating oil composition has a kinematic viscosity at 40° C. of no more than 25 mm²/s.
 2. The lubricating oil composition for a continuously variable transmission according to claim 1, further comprising: (E) a thiadiazole compound in an amount of 180 to 900 mass ppm in terms of sulfur on the basis of the total mass of the composition, wherein a ratio (B+P)/S of a sum of a boron content B (unit: mass ppm) in the composition derived from the component (B) and a phosphorus content P (unit: mass ppm) in the composition to a sulfur content S (unit: mass ppm) in the composition derived from the component (E) is 1 to
 3. 3. The lubricating oil composition for a continuously variable transmission according to claim 1, the (A) lubricant base oil comprising: (Al) a base oil having a kinematic viscosity at 100° C. of no more than 2.8 mm²/s, a viscosity index of no less than 110, and % C_(P) of no less than 90, in an amount of 30 to 100 mass % on the basis of the total mass of the lubricant base oil, the (A) lubricant base oil optionally comprising: (A2) an API group III base oil or group IV base oil or mixture thereof, having a kinematic viscosity at 100° C. of 3 to 10 mm²/s, in an amount of no more than 70 mass % on the basis of the total mass of the lubricant base oil; and a base oil other than the base oils (Al) and (A2), in an amount of no more than 4 mass % on the basis of the total mass of the lubricant base oil, wherein the (A) lubricant base oil has a kinematic viscosity at 100° C. of no more than 3.4 mm²/s.
 4. The lubricating oil composition for a continuously variable transmission according to claim 1, wherein the component (B) is at least one borate ester compound represented by the following general formula (1):

wherein in the formula (1), R¹ is a hydrocarbyl group having a carbon number of 1 to 30; and R² and R³ are each independently a hydrogen atom or a hydrocarbyl group having a carbon number of 1 to
 30. 5. The lubricating oil composition for a continuously variable transmission according to claim 2, the (A) lubricant base oil comprising: (A1) a base oil having a kinematic viscosity at 100° C. of no more than 2.8 mm²/s, a viscosity index of no less than 110, and % C_(P) of no less than 90, in an amount of 30 to 100 mass % on the basis of the total mass of the lubricant base oil, the (A) lubricant base oil optionally comprising: (A2) an API group III base oil or group IV base oil or mixture thereof, having a kinematic viscosity at 100° C. of 3 to 10 mm²/s, in an amount of no more than 70 mass % on the basis of the total mass of the lubricant base oil; and a base oil other than the base oils (Al) and (A2), in an amount of no more than 4 mass % on the basis of the total mass of the lubricant base oil, wherein the (A) lubricant base oil has a kinematic viscosity at 100° C. of no more than 3.4 mm²/s.
 6. The lubricating oil composition for a continuously variable transmission according to claim 2, wherein the component (B) is at least one borate ester compound represented by the following general formula (1):

wherein in the formula (1), R¹ is a hydrocarbyl group having a carbon number of 1 to 30; and R² and R³ are each independently a hydrogen atom or a hydrocarbyl group having a carbon number of 1 to
 30. 7. The lubricating oil composition for a continuously variable transmission according to claim 3, wherein the component (B) is at least one borate ester compound represented by the following general formula (1):

wherein in the formula (1), R¹ is a hydrocarbyl group having a carbon number of 1 to 30; and R² and R³ are each independently a hydrogen atom or a hydrocarbyl group having a carbon number of 1 to
 30. 